The present invention relates to optical trackers and more particularly to a tracker for continuously tracking lateral movements of a person's eye.
This invention was conceived during the development of a system for tracking the pupil of a person's eye while both the head and eye are in motion. Prior eye-gaze trackers are disclosed in U.S. Pat. Nos. 3,864,030 to Cornsweet; 4,287,410 and 4,373,787 to Crane et al.; 4,648,052 to Friedman et al.; and in certain U.S. patent applications of Thomas E. Hutchinson, Ser. Nos. 07/086,809; 07/267,266, filed Nov. 3, 1988 now U.S. Pat. No. 4,836,670; and 07/326,787 now U.S. Pat. No. 4,973,143. Those systems typically illuminate the eye with infrared light which is reflected from various parts of the eye, particularly the cornea and pupil, to an imaging device such as a videocamera. The spatial relations between the corneal and pupil reflections are used to determine the gaze point. For example, the corneal reflection moves about eighty micrometers per degree of eye rotation with respect to the center of the pupil reflection.
It will be understood, however, that unless a corneal reflection tracker is head-mounted or the head restrained so that only in-socket eye rotations are detected, lateral motions of the head (and eye) can generate large errors, e.g., one degree for each eighth-millimeter, in the calculated gaze point. Sophisticated image processing can compensate for such lateral motions, but only to an extent limited by, among other factors, the instantaneous field of view of the camera. Since many potential users of eye-gaze trackers cannot completely control the positions of their heads, means are usually provided to move the field of view of the camera so as to track head motions.
To obtain clear, non-blurred images of an eye in high speed motion, the tracker must be capable of high angular accelerations and velocities. Because of the large moment of inertia of the typical camera-lens assembly used for imaging the person's eye, it is impractical to rotate the camera itself. By placing the camera 90 degrees to the side and putting a mirror at 45 degrees in front of the lens to reflect the camera's instantaneous field of view toward the person's eye, high speed eye tracking can be accomplished by rotating the lower-moment-of-inertia mirror assembly rather than rotating the camera-lens assembly.
We began by considering a conventional double gimbal mount for the mirror control assembly. While the moment of inertia for the inner gimbal was sufficiently low to achieve the desired high-speed, high-acceleration response, the inertia of the outer gimbal, which included the mass of the inner motor and frame, was still too high.
Other representative mechanisms for rotating mirrors are disclosed in U.S. Pat. Nos. 4,811,619 to Cutburth; 4,782,474 to Arai et al.; 4,410,233 to Gerhardt et al.; and 4,315,610 to Malueg. In eyegaze tracking technology, U.S. Pat. Nos. 3,804,496; 4,287,410; and 4,373,787, all to Crane et al., describe rotating mirror systems for tracking eye rotations and translations. The later two patents to Crane et al. disclose the use of gimballed mirrors that are pivoted at their centers so as to rotate about their central vertical and horizontal axes. The earlier Crane et al. patent discloses a small, i.e., less than about 2-inch diameter, two dimensionally gimballed mirror. The mirrors for the three Crane et al. patents are disposed in imaging optical systems at real image points and controlled with position-based servomechanisms in which the error signals are derived from motions of Purkinje images on simple quadrant detectors.
The prior mechanisms are generally more complex and expensive than necessary for efficient head tracking. They tend either to have high inertia, limiting their speed and range of response as described above, or to require special location, limiting their use with simple optics. Furthermore, the prior fast response mirrors such as those of Crane et al. are small so as to achieve the 100-Hz responses needed for tracking in-socket eye rotations, and typically use position-based servomechanisms to maintain the image of the eye at a fixed detector location.